Fibrotic lung disease is characterized by the excessive proliferation and biosynthetic activity of interstitial alveolar fibroblasts. The goal of this project is to test a hypothesis that may bear directly on this disease. The hypothesis states that there are present in the lung subpopulations of fibroblasts, or populations representing stages in fibroblast differentiation, and these populations have unique functional characteristics. Identification of such populations would facilitate the development of simplified experimental approaches to the study of fibrotic lung disease. It is our intention to utilize the monoclonal antibody technology to test this hypothesis. Four specific aims outline the strategy: 1. Produce a library of monoclonal antibodies to fibroblasts from normal and fibrotic rat lung that will distinguish subpopulations of fibroblasts. 2. Isolate fibroblast subpopulations utilizing the monoclonal antibodies. 3. Determine the "clonal stability" of the isolated subpopulations. 4. Ascertain the functional characteristics of the subpopulations with special reference to how they might contribute to pulmonary fibrosis.